Tracheal tube cuff inflation control and monitoring system

ABSTRACT

A tracheal tube is equipped with two air lines to the inflatable cuff, one for monitoring cuff pressure and another of larger diameter for inflating the cuff. Intra-cuff pressure monitored via the smaller diameter line is continuously displayed. A cuff pressure control system allows independent adjustment of cuff pressure via the larger diameter line from high pressure during inspiration to low pressure during expiration. The switchover point is determined by a threshold detecting pressure monitor in the mechanical ventilator inspiration line. The circuit also adjusts automatically to changes in the baseline expiration pressure. The output of a bidirectional mass flow sensor tracks airflow in the cuff inflation line and is integrated to indicate volume. Volume into the cuff is then compared to volume out of the cuff to detect cuff leakage.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional of application Ser. No. 07/701,450, filed May 15,1991 now U.S. Pat. No. 5,235,973.

The subject matter of this application is related to that of U.S. Pat.No. 4,924,862 issued May 15, 1990 to Gary Levinson, entitled "PressureController and Leak Detector for Tracheal Tube Cuff", which isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The invention relates to the controlled inflation of balloon cuffs whichsurround the end of tracheal tubes used in respiratory medicine.Tracheal tubes include both tracheostomy and endotracheal tubes. Anendotracheal tube is one which can quickly be inserted through the mouthor nose into the trachea, while a tracheostomy tube must be insertedthrough a surgical opening in the neck. However, for the purposes ofthis invention, they may be viewed as the same, and will thereforeherein be referred to collectively as tracheal tubes, unless otherwisestated.

Tracheal tubes may be inserted for a variety of reasons, including, theneed for mechanical ventilation, bypass of an obstruction, removal ofsecretions, easier ventilation due to less dead space. In mostcircumstances, it is necessary to seal the outside of the tracheal tubeto the inner tracheal lining, i.e., the tracheal mucosa. Duringmechanical ventilation, this is particularly true, since a closedcircuit is necessary for a ventilator to force a given volume of air oroxygen under pressure into a patient's lungs. When a patient is notbeing mechanically ventilated, a seal may or may not be required. Inthis situation, the need for a seal will generally depend on whether ornot there is a risk of aspiration. In the case of an endotracheal tube,this risk is always present, while with a tracheostomy tube, this riskis often present. Therefore, most patients require a seal either toprevent aspiration or to create a closed circuit for mechanicalventilation. Because of the pressurization of the system, a tighter sealis necessary during mechanical ventilation than is necessary to preventaspiration in the absence of mechanical ventilation.

It is well known that over-pressurization of the tracheal cuff can causesignificant tracheal damage including hemorrhage, ulcers, perforation,and strictures. It is generally accepted that the main cause of thisdamage is occlusion of blood vessels leading to loss of blood flow withresultant necrosis of the tracheal lining. Experience has shown that anintra cuff pressure of less than 25 cm H₂ O is associated withsignificantly fewer complications. This is consistent with experimentaldata suggesting that the capillary perfusion pressure in the trachealmucosa is in the range of 30-40 cm H₂ O. Thus, a cuff pressure of 25 cmH₂ O would still allow some blood flow. However, the numbers noted aboveare not absolute. Damage is occasionally seen with cuff pressures of 25cm H₂ O because perfusion pressures may be lower than expected due, forexample, to low blood pressure. Therefore, the best approach is to usethe lowest cuff pressure consistent with an adequate ventilation sealand the prevention of aspiration.

Tracheal cuffs are frequently positional, that is, as the patient moves,the balloon moves into tighter and looser positions within the trachea.This causes the pressure in the cuff to increase and decreaserespectively. The result is either too much pressure on the mucosa or apressure leak. The problem of tracheal tube movement is not just atheoretical one. The phrase "positional tube" or "positional cuff" areones frequently heard in practice to report a condition in which, afterproper inflation of a cuff, movement of the patient causes excessive,often audible leakage of the ventilator volume around the cuff. In thiscase, the patient's lungs fail to receive the prescribed volume. Inaddition, the loss of a seal between the cuff and trachea allowsaspiration to occur.

A further cause of loss of a seal with resultant aspiration and leakageof ventilator volume is a leak in the cuff itself or in the cuffinflation line.

Problems relating to tracheal cuff pressure control and monitoring havebeen the subject of numerous articles in the medical literature over theyears. To summarize, the problem is to achieve cuff pressures highenough to prevent aspiration and loss of ventilator volume withoutcausing any damage to the tracheal mucosa. Solving this problem has beenparticularly difficult because of the wide range of clinical conditionsin which the cuff system is required to function. For example, in thesetting of an irregular trachea, stiff lungs requiring high pressures toventilate, and low blood pressure, the minimum cuff pressure needed toprevent loss of ventilator volume might cause mucosal damage.

Over the years numerous tracheal tubes and cuff pressure controllershave been designed and patented for the purpose of improved cuffpressure control. To date no completely satisfactory system has beendesigned which solves the problems of tracheal cuff control andmonitoring.

The tracheal tubes have one thing in common, namely, they all have oneinflation line communicating with the cuff for the purpose of inflatingit. Several tracheal tubes have been designed with two cuffs. However,each cuff has a single inflation line. There are two major problems withthe existing tracheal tubes. In the first place, existing inflationlines are of a small diameter ranging from 0.015"-0.030". While thisdiameter is satisfactory for slowly inflating a cuff, it is inadequatefor rapid, synchronous inflation and deflation of a tracheal cuff.Because of the small diameter, the high resistance to flow does notallow rapid enough inflation and deflation of the tracheal cuff at thelow pressures which are used for tracheal cuff inflation. The secondproblem is that it is not possible to synchronously inflate and deflatea cuff, and continuously monitor cuff pressure through a singleinflation lumen. This is because the resistance of the inflation lumencauses significant pressure differential between the external appliedpressure and the actual intracuff pressure. The greater the flow throughthe inflation lumen, the greater the difference between applied pressureand actual pressure in the cuff.

In addition to numerous tracheal tube designs, a number of tracheal cuffcontrollers have been designed, for the purpose of more accuratelyapplying pressure to the tracheal tube cuff. These systems havegenerally over-pressurized cuffs for different reasons. In most casesthey have not allowed continuous bidirectional cuff pressure control. Inother cases, the application of PEEP was communicated to the cuffpressure, while in other cases too much baseline pressure was applied tothe cuff.

In my previous patent (U.S. Pat. No. 4,924,862) I describe a "PressureController and Leak Detector for Tracheal Tube Cuff". That design solvedmany of the problems of tracheal cuff control. I herein describe a newtracheal tube to be used in association with a significantly improvedPressure Controller and Leak Detector.

SUMMARY OF THE INVENTION

A general feature of the invention is a tracheal tube with two separateand distinct tubules entering the cuff. The first tubule is a cuffmonitoring line of standard diameter, while the second tubule is a cuffinflation line of a significantly larger diameter than is generallyfound in tracheal tubes.

Another general feature of the invention is a bidirectional continuousinflation control system for a tracheal tube cuff including a pressurerelief valve having an inlet, an outlet and an exhaust port urged towardthe closed condition, and a cuff inflation line connecting the inlet andoutlet of the relief valve in series between a source of pressurized gasand the cuff. The cuff inflation line connects to the cuff by way of thelarger diameter tubule entering the cuff of said tracheal tube.

Two pressure relief valves are preferred: a low pressure valve in serieswith a high pressure valve in the cuff inflation line. Each valve has aninlet and an outlet and a valved exhaust port urged toward the closedcondition, the high pressure valve requiring higher pressure in the cuffinflation line to open the exhaust port than the low pressure valve. Theinlets and outlets of both valves are connected in series in the cuffinflation line. This arrangement offers the capability of adjusting thepressure in the cuff according to either of two limits whilecontinuously maintaining the ability to supply additional volume to thecuff. Thus, during inhalation, the cuff pressure can be regulated by thesetting of the higher pressure relief valve by, in effect, disabling thelow pressure relief valve.

A further general feature of the invention is the continuous monitoringand display of intra-cuff pressure. This is achieved by the connectionof a pressure transducer to the first, smaller, or monitoring tubulewhich enters the cuff. In the preferred embodiment cuff pressure wouldbe instantaneously and continuously displayed such as on a CRT or LEDBar Graph.

When the tracheal tube is connected to a mechanical ventilator, anotherfeature of the invention is brought into play, in which the exhaust portof the aforementioned low pressure relief valve, which is in series withthe high pressure relief valve, is blocked. In the preferred embodimentsof the invention, a pressure transducer continuously monitors theventilator pressure in the inspiratory tubing and when a specificpressure threshold is reached a solenoid is actuated which blocks theexhaust port of the low pressure relief valve.

In the preferred embodiment, the low and high pressure valves aremagnetic PEEP type valves as hereinafter described.

A further feature of the invention is to monitor cuff leaks by means ofa bidirectional flow detector which produces an electrical output signalindicative of the magnitude and direction of airflow in the cuffinflation line. In the preferred embodiment, flow is integrated toindicate volume. Volume into the cuff is then compared to volume out ofthe cuff to determine if there is a cuff leak. An alarm circuit signalsa leak if volume into the cuff exceeds volume out of the cuff. In thepreferred embodiment, flow is measured by means of a mass flow sensor.

The features of the invention provide continuous instantaneousmonitoring of cuff pressure, automatic bidirectional pressure control ofthe tracheal tube cuff during both respiratory phases, as well as cuffleak protection.

Other advantages and features will become apparent from the followingdescription of the preferred embodiments and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings are briefly described as follows:

FIG. 1 is a block diagram showing the external connections between thetracheal tube, the cuff pressure controller, and a mechanicalventilator, according to the invention.

FIG. 2A is a diagrammatic side view of the tracheal tube of FIG. 1.

FIG. 2B is a diagrammatic cross-sectional view of the tracheal tubetaken along lines A--A of FIG. 2A.

FIG. 3 is a schematic diagram of the internal pneumatic and electricaldesign of the cuff pressure controller of FIG. 1.

FIG. 3A and FIG. 3B comprise an electrical schematic diagram of thethreshold control circuit of FIG. 3.

FIG. 4 is a sectional view of a PEEP valve of the type employed in thecontroller of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a schematic diagram showing the external connections betweenthe tracheal tube 9, the cuff pressure controller-monitor 11(hereinafter referred to as the controller), and a mechanical ventilator18. A source gas is supplied through fitting 13. Since the pressurestandard in hospitals is 50 psi, the controller 11 has been designed toaccept source gas of that pressure. Fitting 14 is the cuff inflationline and connects via external tubing 8 to fitting 7 of tracheal tube 9.This line is for the purpose of synchronously and rapidly inflating anddeflating the tracheal cuff 10 mounted on the distal end of trachealtube 9. Fitting 15 of controller 11 connects via external tubing 3 tofitting 2 of tracheal tube 9. This line is for the purpose ofcontinuously and instantaneously monitoring the actual pressure intracheal cuff 10. Fitting 16 of controller 11 connects via externaltubing 17 to inspiratory tubing 4 of mechanical ventilator 18. Thisconnection allows the controller 11 to continuously monitor the pressurein the inspiratory tubing 4 which connects via trach tube 9 to thepatient's airway. This continuous monitoring allows the controller 11 tosynchronize itself with the phases of ventilation.

FIG. 2 and 2B illustrate the tracheal tube 9. The tracheal tube isfitted with two separate and distinct tubules entering the cuff 10. Eachtubule consists of both a section which is formed within the wall of thetracheal tube and a section which is external to the main body of thetracheal tube 9. As shown in FIG. 2A, fitting 7 will receive theinflation line coming from the controller 11. This fitting is in gaseouscommunication with tubule 6, which is in gaseous communication withtubule 20. Tubule 20 represents the portion of the inflation line whichis formed within the wall of the tracheal tube 9. To allow rapidinflation and deflation of the tracheal cuff, the entire inflation linetubule preferably has a cross-sectional area of approximately six timesthat of the standard tracheal tube. In the cylindrical state this wouldrequire a diameter of approximately 21/2 times that of the standardtubule or a diameter of 0.060-0.080". This is not a problem for tubule6, which is external to the wall of the tracheal tube 9. However, acylindrical diameter of 0.060-0.080" within the wall of tracheal tube 9would not be possible, because that is the approximate wall thickness ofthe tracheal tube itself. Therefore, as noted in FIG. 2B, tubule 20would have to assume either an oval or crescent shape in cross-sectionin order to achieve the required cross-sectional area of about six timesthe standard tubule.

As shown in FIG. 2A, fitting 2 will receive the monitoring line from thecontroller 11. This fitting is in gaseous communication with externaltubule 1, tubule 21, and cuff 10. Because there is insignificant netflow in the monitoring line, there is no requirement for this to be ofan increased cross-sectional area. Therefore, this line may be of thestandard diameter. Section 21 is formed within the wall of the trachealtube 9, while section 1 is external to the body of the tracheal tube 9.In the preferred embodiment, fitting 2 would be fashioned with aspring-loaded plug (not shown) as currently exists in most trachealtubes in use today, such as the endotracheal tube made by Portex,catalog No. 197080. These plugs prevent flow in and out of the cuff. Thefitting 2 contains a female intravenous tubing taper. When a maleintravenous tubing taper is formed into this fitting, the plug isunseated, so that bidirectional flow can occur. This design is typicalof tracheal tubes currently in use. However, fitting 7, which would alsohave a female intravenous tubing taper, would contain no spring-loadedplug. This is because a spring-loaded plug would cause restriction torapid flow. Instead, the open end of fitting 7 would be fitted with asmall removable male plug 22 (FIG. 2A) which would keep this fittingclosed. In that way, this tracheal tube can be used without thecontroller 11 (FIG. 1), as if it were a single line tracheal tube suchas is commonly in use today. In that case the cuff would be manuallyinflated through monitoring line i and 21. To connect the tracheal tube9 to the controller 11, one would merely have to remove the plug 22 fromfitting 7 and connect the inflation line 8 as indicated in FIG. 1.

FIG. 3 is a schematic diagram of the internal pneumatic and electricaldesign of the controller 11 seen in FIG. 1. Along the top of the diagramare seen the four external connections noted in FIG. 1, plus a fifthtube which is the exhaust port (not shown in FIG. 1). A 50 psi sourcegas is connected to fitting 13. The gas flows within tube 29 throughfilter 36, preferably 43 microns, and restrictor 37. In line filters andrestrictors are available from Air Logic, Inc. Restrictor 37, preferably0.007 to 0.010 inch, limits the maximum flow into the system. A flow of1-2 liters per minute is most desirable. Flow then continues within tube29 through pressure controllers 32 and 33, through solenoid 31 shown inits at-rest position, through flow transducer 39, such as AWM2300V fromthe Microwswitch Division of Honeywell, Inc., through solenoid 35 shownin its at rest position, out fitting 14. As indicated in FIG. 1, fitting14 is connected to the inflation line tubule by way of external tube 8and fitting 7. Therefore, in FIG. 3, tube 29 represents the main path ofsupply gas in and out of the cuff.

The operation of pressure controllers 32 and 33 will now be discussed.Positive end expiratory pressure (PEEP) valves are normally used toapply PEEP to a patient's lungs in order to improve oxygenation. Theyprovide back pressure or resistance to exhalation so as to maintaintherapeutic pressure on the lungs. Thus it is not uncommon to find aPEEP valve with just an inlet and an exhaust port in the expiratorytubing (5 in FIG. 1).

As shown in FIG. 3, a pair of modified PEEP type valves, 32 and 33, areemployed in a different way in the controller 11. In essence, thesevalves are adjustable pressure relief valves. They maintain theirpressure by actually releasing or bleeding off volume when the presetpressure is exceeded. Unlike the conventional usage in the expiratoryline, here the PEEP type valves are connected with inlets and outlets inseries with the cuff inflation line 29. Because of the bidirectionalnature of this control system, the pressures will always end up atexactly the preset point, whether the cuff pressure begins above orbelow that point. The PEEP valves thus can instantaneously supply eitherextra volume to, or accept extra volume from, the cuff. Either magneticor spring-loaded PEEP valves may be used, though the design hereindescribed uses magnetic ones, e.g. of the type manufactured byInstrumentation Industries, Inc., of Bethel Park, Pa.

FIG. 4 shows the functional design of a magnetic PEEP valve as appliedto the controller 11. Gas enters port 73 at a rate of 1-2 liters perminute and at a high pressure. Knob 70 turns magnet 71 via thread 72closer to or further from the diaphragm 75. Diaphragm 75 is usuallyferrous metal embedded in rubber. The closer magnet 71 is to thediaphragm 75, the tighter the diaphragm 75 is pulled against orificeflange 77, and the greater the pressure needed to unseat the diaphragm75. When the pressure in chamber 78 reaches the preset point determinedby the position of magnet 71, the excess pressure unseats diaphragm 75and gas exhausts out port 76. Port 74 ultimately connects to thetracheal cuff 10. Once the system is pressurized, there is usually nonet flow out port 74. Therefore, the pressure in chamber 78 is equal tothat of the cuff 10. If the cuff pressure falls, chamber 78 will supplyjust enough volume to port 74 from inlet 73 to equalize the pressure inthe cuff to that of chamber 78. If the cuff pressure rises, chamber 78will accept extra volume from port 74 and exhaust it out port 76 untilthe pressure in the cuff is equalized to that of chamber 78.

Referring back to FIG. 3, low pressure control 32 has a range of 0-35 cmH₂ O. Low pressure control 32 is always adjusted to a lower pressurethan high pressure control 33. Therefore, under normal circumstances,low pressure control 32 determines system pressure, since volume isexhausted out the exhaust port of low pressure control. All of theexhaust gas exiting low pressure controller 32 enters line 56, travelsthrough solenoid 28 shown in its at-rest condition, enters exhausttubing 30 and exhausts out fitting 27 to atmosphere. When solenoid 28 isactuated, tube 56 is closed essentially blocking the exhaust port of lowpressure control 32. This essentially renders low pressure control 32non-functional and the source gas merely passes through it and entershigh pressure control 33. System pressure then rises quickly, above thatset by low pressure control 32, until it reaches a pressure high enoughto unseat the exhaust diaphragm of high pressure control 33. Thispressure is determined by the position of the magnet as described aboveunder the discussion of the PEEP valve shown in FIG. 4. The setting ofhigh pressure control 33 then determines the system and therefore thecuff pressure. Exhaust gas from high pressure controller 33 enters theexhaust tube 30 and exhausts out of fitting 27 to atmosphere. Whensolenoid 28 is again returned to its at rest position, low pressurecontroller 32 becomes functional and again determines system, andtherefore cuff pressure. The controller is designed so that low pressurecontrol 32 determines cuff pressure during expiration while highpressure control 33 determines cuff pressure during inspiration.Pressure gauge 38 is a mechanical pressure gauge and is used to set bothpressure control points.

External fitting 16 connects to the inspiratory tubing of the ventilatorcircuit by way of external tubing 17 of FIG. 1. Therefore, pressuretransducer 53 continuously monitors the pressure within the inspiratorytubing which connects to the patient's airway. The electrical output ofpressure transducer 53 is connected to the threshold control circuit 52.The output of threshold control circuit 52 connects to solenoid 28.Threshold control circuit 52 determines the inspiratory tubing (4 ofFIG. 1) pressure, at which solenoid 28 opens and closes. This thresholdpoint is adjustable by the user via potentiometer 56, which is mountedon the front panel of the controller 11. Therefore, the user candetermine the point in the inspiratory cycle of the ventilator (18 ofFIG. 1) at which the cuff pressure is changed from low pressure controlto high pressure control. The threshold control circuit is designed suchthat positive end expiratory pressure applied to the inspiratory tubing4 via the mechanical ventilator 18 will not affect the set point for theswitching of solenoid 28. This is because the threshold control circuitconsiders the baseline pressure in the inspiratory tubing 4 to be zero,whether that pressure is actually zero or some positive pressure abovezero. A subtraction circuit in the threshold control circuit subtractsthe electrical output of the transducer measured at baseline pressurefrom the electrical output of the transducer continuously. In that way,the application of PEEP and the adjustment of PEEP by the mechanicalventilator will not affect the previous setting of the threshold controlcircuit 52.

A preferred circuit implementation of the threshold control circuit 52is set forth, in FIG. 3A. The positive and negative outputs oftransducer 53 are fed via respective operational amplifiers U16C and Dto differential amplifier U18A, a voltage offset being summed with thepositive input signal by a voltage divider resistor network as shown.The output of amplifier U18A is the instantaneous measurement output or"actual output" corresponding to the sensed pressure. This actual outputis fed to the positive input to differential amplifier U17B, summed withthe output of voltage offset follower U18B and associated circuitry. Thenegative input to differential amplifier U17B is provided by adiode/capacitor peak follower circuit involving operational amplifiersU17A and U18D. Diode D6 causes capacitor C2 to fall back to the baselinevalue quickly after each cycle to keep track of the most recent lowpointof the actual sensed output. Thus the output of the differentialamplifier U17B is the difference between the instantaneous sensedpressure and the current baseline pressure experienced duringexhalation. The output of the differential amplifier U17B is thresholddetected by means of comparator U18C whose reference voltage is set bypotentiometer 56, as shown.

Referring back to FIG. 1, fitting 15 is connected to the cuff monitorline by way of external tube 3. By that means, pressure transducer 49continuously monitors cuff pressure when solenoid 44 is in its at-restposition, as shown in FIG. 3. The electrical output of pressuretransducer 49 is conducted over wire 57 to both a dual comparator 45 andan instantaneous display system 46. In the preferred embodiment, thedisplay system would be a CRT so that real time pressures can be seen.However, an LED bar graph can be utilized satisfactorily. The key factorhere is the need to monitor instantaneous and continuous changes.Therefore, a mechanical pressure gauge would not have a quick enoughresponse time. The pressure display 46, in conjunction with observationsof the patient, is used to help adjust the three determinants of cuffpressure, namely, the low pressure controller 32, the high pressurecontroller 33, and the setting of the threshold control circuit 52 viapotentiometer 56. The dual comparator 45 monitors the output voltage inwire 57, and therefore the pressure in the cuff. If either a preset highor low pressure limit is exceeded, either LED 47 or LED 48 lightsrespectively. The output of dual comparator 45 also enters a time delaycircuit 50 which is set for a 10-15 second delay. If the pressure limitsare exceeded for a period greater than that time, tweeter 49 sounds,indicating too high or too low a pressure. If, on the other hand, theout of range pressure normalizes in less than 10-15 seconds, the timedelay circuit resets itself and the tweeter does not sound. Therefore,the alarm circuit is designed such that the LED's continuously andinstantaneously indicate whether the preset limits are exceeded, whilethe tweeter sounds only if an out of range pressure is detected for aperiod of greater than 10-15 seconds.

Flow transducer 39 continuously monitors flow into the cuff and out ofthe cuff. In the preferred embodiment the flow transducer 39 is a massflow transducer which measures flow bidirectionally. The voltage outputof flow transducer 39 is proportional to the magnitude and direction offlow. This output is conducted via wire 58 to integrator and comparator40. In that circuit the flow is integrated with respect to time, so thatthe volume into and out of the cuff can be measured. Volume out of thecuff is then subtracted from volume into the cuff, and this differenceis displayed on LED bar graph 41. In that way it can be seen by simpleobservation of the LED bar graph whether there is net volume into thecuff. If there is no leak in the cuff or the cuff inflation line, theLED bar graph will read zero net volume into the cuff. If there is aleak in the system, the LED bar graph will gradually indicate excessvolume supplied to the cuff, which will increase continuously over time.This will indicate a cuff leak. Wire 60 connects from the output ofintegrator and comparator 40 to the time delay circuit 50. If a netvolume enters the cuff as indicated on LED bar graph 41, time delaycircuit 50 is actuated and tweeter 49 will sound, if this faultcondition continues. Flow transducer 39 is chosen to have enoughresolution so that net volumes of a few cc's per minute into the cuffcan be detected. In this way, very early cuff leaks can be detected.Because the controller can supply up to 1-2 liters of volume per minute,cuff seal will be maintained after a cuff leak has been detected, sothat an emergency situation should never occur. Only an inordinatelylarge leak would exceed the controller's capacity to maintain cuff seal.However, this degree of leak would most likely develop over a prolongedperiod of time. This would allow sufficient time for the physician toreintubate the patient with a new tracheal tube 9.

The test circuit solenoids will now be described. When solenoid 35 isactuated, the pressure supply line 29 is isolated from the external cuffinflation line, allowing adjustments of both the low and high pressurecontrollers without affecting actual cuff pressure. When the solenoids35 and 34 are actuated, the supply gas is routed from solenoid 35through tube 51, through solenoid 34, into exhaust tube 30, and outexhaust port 27 to the atmosphere. This allows one to test the flowtransducer 39, the integrator comparator 40, the LED bar graph 41, andleak alarm via time delay 50 and tweeter 49. When solenoid 44 isactuated, the cuff monitoring transducer 49 is connected directly to thecuff pressure supply line 29. This allows the pressure reading onpressure gauge 38 to be correlated with the pressure reading on display46. It also allows one to test the high and low pressure alarms. Inorder to test the high and low pressure alarms, both solenoids 44 and 35are actuated, in order to isolate the cuff from the controller. In orderto test the high pressure alarm, solenoid 28 is actuated so that highpressure control 33 determines the system pressure. LED 47 should lightimmediately and tweeter 49 should sound within 10-15 seconds. Lowpressure alarm is then tested by actuating solenoid 31. This has theeffect of venting pressure transducer 49 and pressure gauge 38 toatmosphere through solenoid 31, out exhaust tubing 30, and out exhaustfitting 27 to atmosphere. This should have the effect of immediatelylighting LED 48 and sounding tweeter 49 in 10-15 seconds.

Typical solenoid valves useable in the foregoing system are availablefrom Precision Dynamics, Inc. of New Britain, Conn. Model No. B2215 forthe two-way solenoids 28 and 34 and Model No. B3217 for the remainderwhich are three-way solenoid valves.

The system of the foregoing description has the advantage of offeringflexible operation with automatic, bidirectional control of the cuffinflation line. The use of a low pressure PEEP type valve in series withthe high pressure PEEP type valve insures that the controller is capableof adding volume to the cuff, or exhausting volume from the cuff ineither respiratory phase. In addition, because the cuff line is alwaysconnected to the source gas, volume can be continuously supplied ifthere is a leak, irrespective of the respiratory phase. The ability toadjust the pressure point in the inspiratory tubing at which thecontroller switches between low and high pressure control allows one tovery accurately contour the pressure curve that is applied to the cuff.The ability to accurately monitor not only peak and trough cuffpressures but the contour of the cuff pressure curve is made possible byuse of a new tracheal tube which contains two separate lines into thecuff, one for inflation and one for monitoring. This monitoring isachieved by way of an instantaneous, continuous pressure display on CRTor LED bar graph. This allows both peak and trough and pressure contourcontrol to a degree which has never been possible. Testing of thissystem has clearly confirmed the need for such a delicate control systemsince inordinately wide pressure pulses (pressure pulses of a longerduration than is necessary) and inordinately low pressure troughspreviously unsuspected, have been demonstrated using this system. Boththese conditions can be optimized by combining the continuous monitoringsystem with the adjustment of threshold, and low and high pressurecontrol. The addition of a flow transducer in association with anintegrator, comparator, and LED bar graph, an alarm circuit allows therapid detection of cuff leaks long before any patient morbidity occurs.

The specific implementation of the system given here is only by way ofillustration. Other embodiments are within the claims.

I claim:
 1. An inflation control system comprising:a cuff inflation lineconfigured for sealed, fluidic interconnection with an inflatable cuffof a tracheal tube; an inspiratory tubing monitoring line configured forsealed, fluidic interconnection with inspiratory tubing of a trachealtube; an adjustable high pressure relief valve and an adjustable lowpressure relief valve, both of which have inlet and outlet portsconfigured for sealed, fluidic interconnection in series with a sourceof pressurized gas and said cuff inflation line, both of which also havevalved exhaust ports urged toward the closed condition, a solenoidmounted in series with the exhaust port of the low pressure reliefvalve, said solenoid arranged to block the exhaust port when saidsolenoid is activated; a pressure transducer in sealed, fluidiccommunication with said inspiratory tubing monitoring line to monitorpressure within said inspiratory tubing monitoring line, said pressuretransducer generating an electrical output responsive to pressure withinsaid inspiratory tubing monitoring line; and a threshold control circuitresponsive to said electrical output of said pressure transducer foractivating said solenoid.
 2. The system of claim 1, further comprising auser adjustable potentiometer which sets the pressure point in theinspiratory tubing monitoring line at which said solenoid activates. 3.The system of claim 1 or 2, wherein said threshold control circuitadjusts to varying baseline pressure in the inspiratory tubingmonitoring line during expiration so that changes in baseline pressuredo not cause actuation of the solenoid.
 4. The system of claim 3,wherein said threshold control circuit includes means for subtractingthe electrical output of the transducer measured at baseline pressurefrom the electrical output of the transducer continuously.
 5. The systemof claim 1, further comprising means for detecting and signalling excessvolume entering said cuff inflation line which indicates a cuff leak. 6.The system of claim 5 wherein said detecting means includes a mass flowtransducer which produces a voltage output proportional to the degreeand direction of flow into said cuff inflation line and flow out of saidcuff inflation line.
 7. The system of claim 6, wherein said detectingmeans further includes means for integrating the electrical output ofthe mass flow transducer to determine volume into said cuff inflationline and volume out of said cuff inflation line, whereby volume out ofsaid cuff inflation line is subtracted from volume into said cuffinflation line, and in which net volume into said cuff inflation lineindicates a cuff leak which sounds an alarm.
 8. An inflation controlsystem comprising:a cuff inflation line configured for sealed, fluidicinterconnection with an inflatable cuff of a tracheal tube; an assemblyhaving an exhaust port vented to atmosphere, said assembly configuredfor sealed, fluidic interconnection in series with said cuff inflationline and a source of gas at a pressure greater than atmosphericpressure, wherein in a first state of said assembly said cuff inflationline is coupled to said source of gas to deliver volume into said cuffinflation line, and wherein in a second state of said assembly said cuffinflation line is coupled to said exhaust port of said assembly todeliver volume out of said cuff inflation line; a system for detectingnet volume entering said cuff inflation line by evaluating volume intosaid cuff inflation line and volume out of said cuff inflation line; andan alarm indicating a cuff leak in response to said system detecting netvolume entering said cuff inflation line.
 9. The system of claim 8,wherein said detecting system includes a mass flow transducer whichproduces a voltage output proportional to the degree and direction offlow into said cuff inflation line and flow out of said cuff inflationline.
 10. The system of claim 9, wherein said detecting means furtherincludes means for integrating the voltage output of the mass flowtransducer to determine volume into said cuff inflation line and volumeout of said cuff inflation line, whereby volume out of said cuffinflation line is subtracted from volume into said cuff inflation line,and in which net volume into said cuff inflation line indicates a cuffleak which sounds an alarm.
 11. A cuff inflation methodcomprising:providing a tracheal tube having an inflatable cuff andinspiratory tubing; providing a source of pressurized gas; providing ahigh pressure relief valve and a low pressure relief valve, each of saidrelief valves having an inlet port, an outlet port, and a valved exhaustport; connecting said inlet and outlet ports of said high pressurerelief valve and said low pressure relief valve in series with saidsource of pressurized gas and said inflatable cuff of said trachealtube; supplying pressurized gas from said source of gas to saidinflatable cuff through said high pressure relief valve and said lowpressure relief valve; and opening said valved exhaust port of said lowpressure relief valve in response to both pressure in said inflatablecuff and pressure in said inspiratory tubing of said tracheal tube. 12.The method of claim 11 wherein said valved exhaust port of said lowpressure relief valve opens in response to pressure in said inflatablecuff only when pressure in said inspiratory tubing is below apredetermined level.
 13. The method of claim 11 and comprising thefurther step of opening said valved exhaust port of said high pressurerelief valve in response to pressure in said inflatable cuff.
 14. Themethod of claim 13 wherein said valved exhaust port of said highpressure relief valve opens when pressure in said inflatable cuff isabove a predetermined level.
 15. The method of claim 12 or 14 comprisingthe further step of varying said predetermined level.
 16. The method ofclaim 11 and comprising the further steps of:detecting net volumeentering said inflatable cuff by evaluating volume entering saidinflatable cuff and volume exiting said inflatable cuff; and providingan alarm indication when net volume entering said inflatable cuff isdetected by said detecting step.
 17. A cuff inflation method comprisingthe steps of:providing a tracheal tube having an inflatable cuff;supplying gas to said inflatable cuff of said tracheal tube whenpressure in said inflatable cuff is below a first predetermined leveland pressure in said tracheal tube is above a second predeterminedlevel; and removing gas from said inflatable cuff when pressure in saidinflatable cuff is above a third predetermined level and pressure insaid tracheal tube is below said second predetermined level, whereinsaid third predetermined level is less than said first predeterminedlevel.
 18. A respiration system comprising:a tracheal tube having aninflatable cuff and inspiratory tubing; an adjustable high pressurerelief valve and an adjustable low pressure relief valve, both of whichhave inlet and outlet ports configured for sealed, fluidicinterconnection in series with a source of pressurized gas and saidinflatable cuff of said tracheal tube, both of which also have valvedexhaust ports urged toward the closed condition; a solenoid mounted inseries with the exhaust port of the low pressure relief valve, saidsolenoid arranged to block the exhaust port when said solenoid isactivated; a pressure transducer in sealed, fluidic communication withsaid inspiratory tubing of said tracheal tube to monitor pressure withinsaid inspiratory tubing, said pressure transducer generating anelectrical output responsive to inspiratory tubing pressure; and athreshold control circuit responsive to said electrical output of saidpressure transducer for activating said solenoid.